1. Field of the Invention
The present invention generally relates to transfers of data between host computers and peripheral devices and more particularly relates to such transfers which utilize a fiber optic transmission medium.
2. Description of the Prior Art
It has been known for some time to transfer digital data between peripheral equipments and host computers. The earliest and most popular medium for such transfers is electrical energy, which flows within a circuit including portions of the host computer, peripheral devices, and interconnecting electrical cables. These data transfers may be serial in nature, which transmit one data bit at a time, or parallel, in which a number of data bits are transmitted simultaneously.
For a given switching speed of an individual electrical circuit, the parallel approach is inherently faster because it transfers multiple data bits simultaneously. This has meant that serial transmissions have tended to be limited to low data rate information paths or to long distances wherein the cost for parallel transmission is prohibitively expensive. Therefore, most modern day data transfers between host computers and associated peripheral devices utilize parallel electrical transmission. A typical protocol for such transmissions is the popular Block Multiplexer Channel (i.e. BMC) utilized by Unisys Corporation. This highly efficient approach transfers data as parallel bytes (along with error detection bits) over a first parallel cable and control signals over a second parallel cable. Because these two cables transfer data and control signals only in one direction, a second pair of cables is usually needed for transfers in the opposite direction. The technique provides an effective transfer rate in each direction of nearly 4.5 MB/sec.
A second medium which is gaining popularity for data transmission is fiber optics. In this approach, the digital data is converted to pulses of light which are transferred over a special light conducting fiber optic cable. Because this transmission medium does not experience the same distributed capacitance which delays electrical transmissions, higher data rates can generally be achieved for a given transmission energy. In fact the data transmission rates tend to be sufficiently high, that serial fiber optic transmissions can be utilized to replace parallel electrical transmissions for many host computer to peripheral device transfers.
However, the conversion of historically parallel electrical transmission paths to newer serial fiber optic transmission paths tends to result in the use of protocols which are different and incompatible with the older parallel systems utilizing the electrical medium. These incompatibilities result from the basic differences in the media, conversion from parallel to serial transmission, and different methods of generating and using control signals. In most cases this means that the advantages of fiber optic transmission tend to be limited to systems which are newly designed around these unique requirements.
For some applications, it is desirable to retrofit existing systems with fiber optic transmission for one or more data links between host computer and peripheral devices. The typical manner of providing such a retrofit is to design and fabricate an interface device for both host computer and peripheral device ends of the transmission path. A general characteristic of such interface devices is that they contain sufficient memory to buffer a complete transmission or logical portion thereof, to permit ease of conversion from the existing electrical protocol to the new fiber optic protocol.
Functionally, a first parallel data transmission is made to the buffer within the interface device from the parallel electrical transmission link. After the data has been stored in the buffer memory, a second serial transmission is made over the fiber optic medium between the buffer memory of the first interface device and the buffer memory of the second interface device. Yet a third data transmission is made from the buffer memory of the second interface device via a parallel electrical cable to the receiving device (i.e. host computer or peripheral device). This results in three separate transmissions of the same data, each with its own control signals and protocol.